This application relates to a shock absorber apparatus for use as part of a tool string for use in bore holes and/or pipelines. This invention particularly though not exclusively relates to a shock absorbing tool for use with perforating guns in the oil well logging industry.
Present day wells for extracting oil, gas or water, are mostly constructed by drilling in the ground and placing a tubular steel casing as a liner in the well. Once the casing is in place, the well may be completed using a perforating gun to form perforations in the casings through which surrounding reservoir fluid, comprising oil, gas and water, may flow inwardly. From this reservoir fluid, the oil, gas or water may then be extracted.
A perforating gun is an instrument equipped with a number of powerful explosives or ‘shape charges’. These are about one inch (25 mm) in diameter, and when detonated produce a plasma which destroys both the steel pipeline casing and typically 1 to 2 feet (300–600 mm) of the surrounding rock. Reservoir fluid from a deposit surrounding the well can then flow through the perforated rock into the pipeline and be pumped away to the surface.
It is desirable to deploy sensors such as temperature and pressure sensors alongside the perforating gun so that data about the ‘perforating’ explosion and the resulting fluid flow may be collected. In this way perforation techniques may be improved.
The perforating gun and sensor tools may be deployed in the bore hole as or as part of a tool string, which is an array of one or more guns or tools connected end-to-end on a wireline or cable. The wireline may provide both electrical power and a possible mechanical drive means for the tool string in the bore hole.
The blast from the shape charges of the perforating gun is powerful enough to rip through both the steel casing and enough of the surrounding rock to release the reservoir fluid. The blast therefore poses a significant risk of damage to the sensors that accompany the perforating gun into the bore hole. Such sensors often employ photo-multipliers, gamma ray detectors and circuit boards which could easily be damaged by the shock wave from the explosion. For this reason, it is desirable to separate the tool sensors and perforating gun from one another on the tool string by a shock absorbing tool. A good shock absorber is important as it means that the sensor tools may not only be protected from the shock wave but also may be placed closer to the perforating gun in order to collect data closer to the explosion site.
A number of shock absorbers for use down-hole in an oil well are known and rely on a piston or plunger mechanism to absorb the energy from the shock. Typically the piston or plunger chamber is filled with a fluid which is forced through holes in the piston chamber as the piston is depressed, providing an increasing resistive force in opposition to the force which is depressing the plunger or piston. A problem with shock absorbers of this type is that they are not responsive enough to adequately dissipate the energy released from the explosion. This is because the blast from the shape charges lasts only an instant and produces a shock wave that contains both high and low frequencies. At high frequencies, the piston and fluid arrangement simply cannot react quickly enough and the shock wave may travel through it as if it were a solid. As a result the sensors may be damaged
A prior art shock absorber tool for use in an oil well is described in European patent application 0,414,334. This shock absorber comprises a piston and a fluid filled piston chamber which has a number of openings to allow fluid to escape from the chamber into a surrounding space as the piston is compressed. As the piston moves into the piston chamber the hole area through which the fluid can escape is diminished resulting in an increasing resisting force from the fluid. An additional like shock absorber can be connected in series with it.
European patent application 0,489,527 discloses a further shock absorbing tool which makes use of two telescopic casings disposed one inside the other and defining a space between them which is filled with a compressible oil. Disposed between the two casings is a metering clearance sleeve which defines two spaced sealed voids in this space and through which the oil is caused to pass in response to a shock wave.
U.S. patent application Ser. No. 5,320,169 discloses a gauge carrier having shock absorber means. The shock absorber means comprises an upper set of belleville springs or disc springs mounted on a rod member and contained between an actuator head portion of the rod member and an annular disc support. A lower set of belleville springs is contained between the disc support and a lower connector member. The rod member is screwed into a threaded bore in the connector member.
Further shock absorbing devices are disclosed in U.S. Pat. Nos. 6,109,355 and 4,817,710 which use an elastically deformable body, and resilient contact pads, respectively.
We have appreciated that known shock absorbers, such as those described in the prior art referred to above, cannot respond quickly enough to adequately absorb the energy in a shock wave, and consequently that sensor tools used in conjunction with such shock absorbers and perforating guns are still at risk of damage from detonation of the shape charges.